This invention relates to a process for preparing xcex2-alkoxy-nitriles by reacting low molecular weight xcex1,xcex2-unsaturated nitrites, having up to 40 carbon atoms for example, with monohydric, dihydric or trihydric alcohols, each having a molar mass of up to 2.5xc3x97103 g/mol for example, in the presence of basic catalysts at from xe2x88x9220 to +200xc2x0 C.
The 1,4-addition of monohydric or polyhydric alcohols to xcex1,xcex2-unsaturated nitriles is a known reaction which is classified as a Michael-type addition in J. March, Advanced Organic Chemistry, 3rd Ed., page 665, J. Wiley and Sons, 1985, because of the reaction mechanism.
As observed in H. A. Bruson, Organic Reactions, Vol. 5, Chapt. 2, page 89, R. Adams (Ed.), J. Wiley, 1949, for example, this addition reaction usually requires a basic catalyst in order that satisfactory reaction rates may be obtained.
In many cases, the reaction mixtures comprising the 1,4-addition product are directly, without purification, converted into xcex3-alkoxyamines in a second process step by subsequent catalytic hydrogenation. Existing processes are surveyed, for example, in Houben-Weyl, Methoden der organischen Chemie, volume 11/1, pages 341 et seq., 4th edition (1957).
Since the 1,4-addition of alcohols to xcex1,xcex2-unsaturated nitriles to form xcex2-alkoxynitriles is reversible, the reversal of the formation of xcex2-alkoxynitrile must be avoided in any subsequent hydrogenation in the presence of the basic catalyst (cf. also: B. A. Bruson, Organic Reactions, Vol. 5, page 90, para 3, lines 8-11). Removal prior to the hydrogenation step of the small amounts of basic catalysts used is uneconomical, and they therefore have to be neutralized with an acid. In any subsequent catalytic hydrogenation of the xcex2-alkoxynitrile, the hydrogenation catalyst must not be damaged by the 1,4-addition catalyst or its neutralized form.
Typical catalysts for the 1,4-addition of alcohols to xcex1,xcex2-unsaturated nitriles include, for example, the metals sodium and potassium or their oxides, hydroxides, hydrides, cyanides and amides, as likewise observed in H. A. Bruson, Organic Reactions, Vol. 5, pages 81 and 89. The catalysts are customarily used in amounts of from 0.5 to 5% by weight, based on the alcohol.
W. P. Utermohlen, J. Am. Chem. Soc. 67, 1505-6, disclosed the use of sodium methoxide as basic catalyst.
The use of alkali metals entails appreciable problems with the handling of these reactive catalysts. Furthermore, alkali metal hydrides, amides and alkoxides are highly moisture-sensitive and industrially handleable only at great expense. And the chemical composition of these catalysts must be checked before use to determine their activity.
There has therefore been no shortage of attempts to find catalysts which are simple to handle on an industrial scale and, at the same time, are sufficiently active to enable the 1,4-addition reaction to take place with very high space-time yields.
DE-A-20 61 804 discloses that, inter alia, organic secondary or tertiary amines, for example piperidine or triethylamine, are useful as basic catalysts for the 1,4-addition of B-thio or xcex2-sulfoxide-substituted ethanols to xcex1,xcex2-unsaturated nitriles. However, secondary amines have only limited usefulness as catalysts, since they actually react with xcex1,xcex2-unsaturated nitrites.
DE-A-35 22 906 discloses basic catalysts, including tertiary amines, for example triethylamine or pyridine, useful both for the preparation of 2,2xe2x80x2-dicyanodiethyl ether (NCxe2x80x94(CH2)2xe2x80x94Oxe2x80x94(CH2)2xe2x80x94CN) from acrylonitrile and water and for the synthesis of xcex2-alkoxynitriles from 2,2xe2x80x2-dicyanodiethyl ether and an alcohol.
U.S. Pat. No. 2,333,782 discloses tributylamine as catalyst for the 1,4-addition of formaldehydecyanohydrin to acrylonitrile to form xcex2-(cyanomethoxy)propionitrile.
Basic catalysts used for the 1,4-addition of alcohols to xcex1,xcex2-unsaturated nitriles have frequently been quaternary tetraalkylammonium hydroxides or solutions thereof, for example benzyltrimethylammonium hydroxide (a=Triton(copyright) B), for example described in U.S. Pat. No. 3,493,598 and W. P. Utermohlen, J. Am. Chem. Soc. 67, 1505-6 (1945), or tetrakis(2-hydroxyethyl)ammonium hydroxide, described for example in DE-A-21 21 325 and DE-A-22 17 494.
As is common general knowledge, tetraalkylammonium hydroxides are thermally unstable, decomposing to form a trialkylamine, alkene and water (Hofmann elimination; see for example: R. T. Morrison and R. N. Boyd, Organic Chemistry, 6th Ed., 1992, page 854 bottom to page 855 top).
Tetraalkylammonium hydroxides having from 1 to 4 xcex2-hydroxy substituents are likewise thermally unstable, decomposing by intra- and/or intermolecular reactions (see for example: A. R. Doumaux et al., J. Org. Chem. 38, 3630-2 (1973) and A. C. Cope et al. in xe2x80x98Organic Reactionsxe2x80x99, Vol. 11, Chapter 5, Wiley, New York, 1960).
These catalysts and their solutions therefore have only limited storage life, so that their chemical composition needs to be checked too before use to determine their activity.
Owing to their thermal lability, tetraalkylammonium hydroxides used as catalysts for the 1,4-addition of alcohols to xcex1,xcex2-unsaturated nitrites at the customary reaction temperatures of from 35 to 140xc2x0 C. (H. A. Bruson, Organic Reactions, Vol. 5, Chapt. 2, pages 89, 90 and 93) frequently give poor yields of the 1,4-addition products. Another important disadvantage is the fact that a thermally partially decomposed catalyst or its solution will cause a delay in the startup of the 1,4-addition reaction. This may cause the nitrile concentration in the reaction vessel in which the addition reaction is being carried out by addition of the xcex1,xcex2-unsaturated nitrile, for example acrylonitrile, to the alcohol will build up to a dangerously high level and, in the extreme case, may lead to a markedly thermic polymerization of the xcex1,xcex2-unsaturated nitrile.
Further disadvantages of quaternary ammonium hydroxides are their inutility for the 1,4-addition of polyhydric alcohols to xcex1,xcex2-unsaturated nitrites (see DE-A-22 17 494), the fact that the 1,4-addition products frequently exhibit an undesirable discoloration, and the need to neutralize them with an acid after the 1,4-addition reaction has taken place and to remove the resulting salt if the xcex2-alkoxynitriles are to be subjected directly to a catalytic hydrogenation to form xcex3-alkoxyamines (see DE-A-21 36 884).
It is an object of the present invention to provide an improved process for the 1,4-addition of monohydric, dihydric or trihydric alcohols to xcex1,xcex2-unsaturated nitrites, which does not have the above-described disadvantages and which even makes it possible for the resulting reaction:mixture of the 1,4-addition products to be converted directly in a second process step into xcex3-alkoxyamines by hydrogenation in the presence of a hydrogenation catalyst without there being a need for any prior removal or neutralization of the catalyst for the 1,4-addition.
We have found that this object is achieved by a process for preparing xcex2-alkoxynitriles:by reacting xcex1,xcex2-unsaturated nitrites, having from 3 to 40 carbon atoms for example, with monohydric, dihydric or trihydric alcohols, each having a molar mass of up to 2.5xc3x97103 g/mol for example, in the presence of basic catalysts at from xe2x88x9220 to +200xc2x0 C., which comprises using a diazabicyclo-alkene catalyst of the formula I 
where from 1 to 4 hydrogen atoms may be independently replaced by the radicals R1 to R4,
in which case R1, R2, R3, R4 are each C1-20-alkyl, C6-20-aryl or C7-20-arylalkyl, and
n and m are each an integer from 1 to 6.
The radicals R1, R2, R3 and R4 independently have the following meanings:
C1-20-alkyl, such as methyl, ethyl, n-propyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, cyclopentyl, cyclopentylmethyl, n-hexyl, isohexyl, cyclohexyl, n-heptyl, isoheptyl, cyclohexylmethyl, n-octyl, isooctyl, n-nonyl, n-decyl, n-undecyl, n-.dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl,
xe2x80x83preferably C1- C8-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl, neopentyl, 1,2-dimethyl-propyl, n-hexyl, isohexyl, sec-hexyl, cyclohexyl, cyclopentylmethyl, n-heptyl, isoheptyl, cyclohexylmethyl, n-octyl, isooctyl,
xe2x80x83particularly preferably C1- to C4-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl,
C6-20-aryl, such as phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl, 9-anthryl preferably phenyl, 1-naphthyl, 2-naphthyl, particularly preferably phenyl,
C7-20-arylalkyl, preferably C7-12-phenylalkyl, such as benzyl, 1-phenethyl, 2-phenethyl, 1-phenylpropyl, 2-phenylpropyl, 3-phenylpropyl, 1-phenylbutyl, 2-phenylbutyl, 3-phenylbutyl and 4-phenylbutyl, particularly preferably benzyl.
The aforementioned radicals may bear substituents that are inert under the reaction conditions, such as one or more alkyl radicals, e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl.
The indices n and m in the formula I are independently integers from 1 to 6. Preferably, both n and m are integers from 1 to 3. Particularly preferably, n is 1, 2 or 3 and m is 2.
Examples of useful catalysts of the formula I are:
1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU), 1,6-diazabicyclo[5.5.0]dodec-6-ene, 1,7-diazabicyclo[6.5.0]tridec-7-ene, 1,8-diazabicyclo-[7.4.0]tridec-8-ene, 1,8-diazabicyclo[7.5.0]tetradec-8-ene, 1,5-diazabicyclo[4.4.0]dec-5-ene (DBD), 1,8-diazabicyclo-[5.3.0]dec-7-ene, 1,10-diazabicyclo[7.3.0]dodec-9-ene, 1,10-diazabicyclo[7.4.0]tridec-9-ene, 2-methyl-1,5-diazabicyclo[4.3.0]non-5-ene, 3-methyl-1,5-diazabicyclo-[4.3.0]non-5-ene, 7-methyl-1,5-diazabicyclo[4.3.0]non-5-ene, 7-benzyl-1,5-diazabicyclo[4.3.0]non-5-ene, 11-methyl-1,8-diazabicyclo[5.4.0]undec-7-ene, 10-methyl-1,8-diazabicyclo-[5.4.0]undec-7-ene, 6-methyl-1,8-diazabicyclo[5.4.0]undec-7-ene, 6-benzyl-1,8-diazabicyclo[5.4.0]undec-7-ene, 2-methyl-1,5-diazabicyclo[4.4.0]dec-5-ene, 3-methyl-1,5-diazabicyclo-[4.4.0]dec-5-ene, 7-methyl-1,5-diazabicyclo[4.4.0]dec-5-ene, 7-benzyl-1,5-diazabicyclo[4.4.0]dec-5-ene.
Preference is given to DBN, DBD and DBU and particular preference to DBU and DBN, since these compounds are readily obtainable.
Similarly, mixtures of compounds of the formula I, for example a mixture of DBU and DBN, are useful as catalysts.
The catalysts of the formula I according to the invention, in contrast to the prior art quaternary ammonium compound catalysts, are also very useful for the complete conversion of all hydroxyl groups of di- or trihydric alcohols by 1,4-addition to xcex1,xcex2-unsaturated nitriles.
A further advantage of the process of the present invention is that the 1,4-addition products, i.e., the xcex2-alkoxynitriles, exhibit significantly less discoloration than from using the prior art quaternary ammonium compound catalysts.
Diazabicycloalkenes of the formula I are preparable by various methods. An example of a known method is the addition of acrylonitrile to lactams to form cyanoethyllactams which are then hydrogenated to aminopropyllactams and finally cyclized with acid-catalyzed water elimination to form the diazabicycloalkenes (H. oediger et al., Synthesis, 591-8 (1972); H. Oediger et al., Chem. Ber. 99, 2012-16 (1966).; L. Xing-Quan, J. Nat. Gas Chem. 4, 119-27(1995)).
Prior German patent application 19752935.6 describes a process for preparing diazabicycloalkenes by reaction of lactones with diamines with water elimination.
Since diazabicycloalkenes of the formula I are strong bases which, owing to their low nucleophilicity with respect to common tertiary amines such as triethylamine or N,N-dimethylaniline, for example, occupy a special position, they are used in a whole series of organic reactions, for example in hydrogen halide elimination reactions (see above-cited references).
The excellent activity of diazabicycloalkenes of the formula I as catalysts for the 1,4-addition of alcohols to xcex1,xcex2-unsaturated nitrites is surprising because it was known from JP-A-5-25201/93 (Example 1) that the reaction of already 85% cyanoethylated pullulan with acrylonitrile (ACN) requires an extremely high excess of about 30 mol of ACN per mole of hydroxyl group, a huge 6% by weight of the catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and also a long reaction time of ten days.
Pullulan is a D-maltotriose polymer having a molar mass within the range from 5xc3x97104 to 2xc3x97106 g/mol (Lit.: A. Jeanes in xe2x80x98Extracellular Microbial Polysaccharidesxe2x80x99 (P. A. Sandford and A. Laskin, Ed.), pages 288, 289 and 292, Am. Chem. Soc., Washington, DC (1977)).